Current state-of-the-art solid-state lighting devices use wavelength conversion materials, e.g. phosphor, in order to convert a fraction of the light of e.g. a blue LED into light of a longer wavelength e.g. red light. The mixing of the light of the blue LED with the phosphor-emitted light results in a spectrum that is perceived by humans as white light.
Although the conversion process is very efficient, phosphor-converted solid-state light sources have a non-directional (Lambertian) light distribution and hence a relatively large étendue. Due to the non-directional light distribution of these light sources it is very difficult to achieve efficient light coupling into light guides thereby making such light sources not or at least less suitable for light guide applications. Moreover, the non-directional light distribution often requires complex secondary optics in order to achieve the desired collimated light beam of a homogenously distributed intensity that is required in e.g. projector lights and headlights spot lights. Besides the complexity, secondary optics are often bulky and inefficient thereby leading to an overall loss of efficiency of the optical system. For example, a relatively complex combination of reflective and refractive optics is needed in order to shape the non-directional light of the phosphor-converted light source into a collimated beam of a homogenously distributed intensity.
Lozano et al., describe in their article “Plasmonics for solid-state lighting: enhanced excitation and directional emission of highly efficient light sources”, Light: Science & Applications (2013)2, e66, the use of a metal nanoparticle array that is in contact with a luminescent wavelength conversion layer in order to achieve enhanced directional emission of light in the forward direction. Such array allows conversion of non-directional light of a solid-state luminescent light source into light with a preference in the forward direction. Although the light originating from such array is directional in the forward direction, it still needs relatively bulky refractive secondary optics in order to achieve a collimated beam of a homogenously distributed intensity. Moreover directional light in the forward direction is not or less suitable for light injection in planar light guides.
Hence, in view of the above, it is desired to provide simple luminescent structures for solid-state light sources that enable efficient directional emission of light in the sideward direction. In particular, it is desired to provide sideward emitting luminescent structures for use in solid-state light sources that can be easily coupled to planar light guides and that can be used with simple reflective light concentrator, e.g. a compound parabolic concentrator (CPC), in order to form high brightness collimated light beams.